Transformation of Amorphous Nanobowls to Crystalline Ellipsoids Induced by Trans-Cis Isomerization of Azobenzene

The stimuli-responsive transition of nanostructures from amorphous to crystalline state is of high interest in polymer science, but is still challenging. Herein, the transformation of amorphous nanobowls to crystalline ellipsoids triggered by UV induced trans-cis isomerization is demonstrated, using an azobenzene-containing amphiphilic homopolymer (PAzoAA) as a building block. The amide bond and azobenzene pendants are introduced to the side chain of PAzoAA to afford hydrogen bonding and pi-pi interactions, which promote the formation of nanobowls rather than spherical nanostructures. Upon exposure to UV irradiation, trans-cis isomerization of azobenzene pendants occurs, leading to the increase of hydrophilicity and destruction of pi-pi interaction, further resulting in the disassembly of the nanobowls. Then the PAzoAA re-assembles to form crystalline ellipsoids instead of amorphous nanostructures when recovered at 70 degrees C without UV light. Further, it is confirmed that the high incubation temperature after UV irradiation is critical for the cis-trans transformation and the high mobility of the polymer chains to facilitate the regular rearrangement of azobenzene pendants. Overall, a facile method to achieve the transformation of amorphous nanobowls to crystalline ellipsoids is proposed, which may bring new insight into preparation of crystalline nanoparticles using amorphous precursors.